1. Field of the Invention:
This invention relates to polyisocyanate prepolymers which are the reaction products of (1) organic polyisocyanate and (2) hydroxyl-terminated digestion products of (a) scrap or waste polyalkylene terephthalate polymers and (b) organic polyols.
2. Description of the Prior Art:
Polyalkylene terephthalate polymers are widely used as films and fibers. In general they have molecular weights greatly in excess of 15,000 and are sometimes referred to as super-polyesters. Numerous processes have been proposed for recovering useful products from waste or scrap polyalkylene terephthalate by depolymerizing or partially depolymerizing the polymer to lower molecular weight recurring structural units having the essential formula ##STR1## wherein --G-- represents a divalent organic radical containing 2 to 10 carbon atoms attached to the adjacent oxygen atoms by saturated carbon atoms. In polyethylene terephthalate, G is --C.sub.2 H.sub.4 --. Super-polyesters have been digested primarily for reuse in the process of making additional super-polyesters, that is, the digestion products are reintroduced into the process from which they originated. Known digestion processes include treatment with caustic monohydric alcohol (U.S. Pat. No. 3,728,287); treatment with monohydric alcohol and ester interchange catalyst (U.S. Pat. No. 3,488,298); heating with glycols and vinyl acetate (U.S. Pat. No. 3,857,799); heating with bis ester solvents (U.S. Pat. No. 3,884,850); dissolving in ethylene glycol and/or terephthalic acid and/or dimethyl terephthalate (U.S. Pat. Nos. 3,907,868; 3,701,741; 3,703,488; 3,776,945; 3,257,335).
The concept of preparing polyurethane elastomers and polyurethane foam products by reaction of polyisocyanates with polyols is well developed.